The present invention relates generally to solar power systems. More particularly, the invention relates to an azimuth drive unit for a solar power system having a multistage chain and sprocket configuration.
High concentration solar power systems typically use reflectors, drive systems, and receivers to provide thermal energy for various industrial and commercial processes. Each reflector transfers solar radiation to a receiver and is typically a parabolic trough, a dish concentrator, or a field of heliostats. The reflected radiation heats a working fluid and drives a power conversion system to produce electricity. Similarly, tracking photovoltaic systems ensure that the solar irradiance incident on the solar arrays meets appropriate optical requirements. In each case, the drive system allows the reflectors or collector to track the sun in azimuth and elevation and is critical to the overall solar power system performance.
As already mentioned, the drive system essentially has the same requirements for all of the above types of reflectors. Accurately tracking the sun requires that the drive system meets exacting requirements under a wide variety of environmental conditions. The drive system must therefore have minimal backlash, high stiffness, and sufficient efficiency to allow for small tracking movements, even in the presence of wind loads associated with wind speeds on the order of 30 to 50 mph (18-31 kmph). Furthermore, the drive system can be subjected to temperatures ranging from lows on the order of 20 to 40xc2x0 F. (xe2x88x926xc2x0 to 4xc2x0 C.) below zero to highs on the order of 130xc2x0 F. (54xc2x0 C.) and above. Other harsh environmental conditions include rain, snow, ice, earthquakes, dust intrusion, corrosion, etc.
As already noted, in virtually all heliostat and dish concentrator designs, the configuration of the drive system is based on the standard elevation and azimuth approach. Similarly, parabolic trough reflectors require the drive system to rotate about its longitudinal axis. Photovoltaic trackers ensure that the solar irradiance incident on the solar arrays meets the appropriate optical requirements and are fundamentally similar to the above discussed systems. For heliostats and dishes, the azimuth drive unit is typically supported by a vertical pedestal, about which the reflector (mounted on the drive unit) rotates. The elevation drive unit can either be another rotational drive unit, or it can be a linear actuator. The azimuth drive unit is commonly the more costly unit. It is therefore desirable to provide a low cost azimuth drive unit that has low backlash, exhibit high stiffness, and allow for sufficiently small tracking movements under harsh environmental conditions.
The primary design requirement for azimuth drive units is that the reduction ratio be sufficient to ensure that tracking error is on the order of 0.1 to 1.0 milliradians for the reflected beam. The specific requirement for tracking accuracy and angular rate depends on the application, but in general, the drive unit itself should be able to maintain pointing accuracies of this magnitude. These pointing accuracies must be with slow rates (approximately 150 per hour), and wind speeds on the order of 30 mph (18 kmph). Higher rotation rates are necessary for emergency slew, start up and shut down, and certain maintenance conditions. In practice, the azimuth drive unit must have an overall reduction ratio on the order of 40,000 to 1. It should be noted that with the high reduction ratio and slow speed, the actual azimuth drive unit horsepower rating is low (typically on the order of 1/50 to 1 horsepower motors), with most applications tending towards fractional horsepower.
Another design requirement is that the azimuth drive unit be able to acquire the sun over a wide azimuthal angular range, since the sun""s position varies significantly over the course of a year. For heliostats and dishes, used for either solar thermal or photovoltaic systems, the elevation angular range is approximately 90 degrees. This is unless inverted stow is used, in which case the range is approximately 180 degrees. There are situations in which the sun""s passage through the sky requires the reflector to move around a so-called xe2x80x9csingularity position.xe2x80x9d This is caused by the sun""s angle and the reflector""s location and aimpoint being such that the reflector would tend to move in an angle that lies below horizontal. In this special case, the reflector is rotated in azimuth 180 degrees or more to easily move to the required position. For the parabolic trough, the rotational range is on the order of 180 degrees, east to west, or less, if the troughs are aligned with the longitudinal axis along an east-west line.
Another design requirement is that the drive units be able to withstand the static and dynamic loads imposed primarily by wind and gravity. Wind-induced dynamic loads (wind gusts) impose particularly severe effects that lead to premature failures. Most of the drive unit failures observed over the years involve sudden shock loads imposed on essentially non-compliant drive unit components, such as gear teeth, etc. In these cases, the need for stiffness (little or no backlash and little compliance) means that there is little or no damping of the imposed loads. Thus, high shock loads may be imposed throughout the drive train.
A number of drive units have been developed that meet the above requirements, but in general, these units are custom designed. Thus, much of the hardware and components are made especially for the specific application. As a result, conventional units are relatively expensive, especially in the low production quantities needed for early market entry. This increased cost is one of the reasons that the high concentration solar thermal energy industry has had difficulty achieving competitively priced electricity. Since the heliostat or dish is typically on the order of one-half the cost of the entire system, and the drive unit cost is typically on the order of one-third to one-half that of the heliostat or collector, the impact of the drive unit cost on the system can be prohibitive. This is especially true for early production rates.
There has been a tendency over the last 25 years among virtually all developers of heliostats to build larger units, resulting in aperture areas of 100 to 150 square meters or higher. This tendency to develop relatively large size heliostats has further increased the difficulty of achieving low cost for early market penetration with relatively low initial production rates. This is especially true for the drive units, since the tooling and factory setup costs are high for these custom designs, and the costs must be amortized over a relatively small number of units. The tooling costs themselves are also higher for larger drive units, In addition, for the same total power capacity needed in the market, fewer units will be produced, which means that the cost improvements occurring as a part of the manufacturing learning curve will be less.
By having smaller heliostat surface areas, and hence lower loads, a larger number of commercial components would be available for potential use, as opposed to developing custom drive units. It can also be shown that the wind induced loads on small heliostats are lower than for large heliostats. This is because there is a boundary layer effect that causes the wind speed to be less within a few meters of the surface, and higher at about 5 meters and up. Thus, small heliostats would have a higher design safety factor, using the same wind speed (typically measured at least 5 meters above ground).
An additional problem with large heliostats is that the optical performance suffers as the aperture area increases. Therefore, a larger total heliostat area is needed for the same size plant output power, which has a substantial effect on the cost of the system. It should be noted that the heliostats are normally built with a range of curvatures, depending on their location in the field, such that they tend to concentrate the sunlight on the receiver. It can be shown by optical analysis codes and theory that for a given size of heliostat field and receiver, increasing the size of the individual heliostats will increase the off-axis aberration, resulting in increased optical losses. For example, it has been shown in ray trace simulations that the optical losses are on the order of 20 percent as the heliostat size is increased from approximately 10 m2 to 50 m2 to 100 m2. Conversely, making the heliostats too small increases the cost associated with field wiring, controllers, and installation. Therefore, there is an optimum reflector size for any given application.
The size of a dish concentrator is determined by the power rating and efficiency of the power conversion unit. For the McDonnell Douglas designed 25 kilowatt electric Dish Stirling concentrator, the aperture area is 90 m2. A similar design in principle was developed under DOE sponsorshipxe2x80x94this unit has a 120 m2 aperture area and approximately a 20 to 25 kilowatts electric output. There are other Stirling units available, however, with approximately 3 to 10 kilowatts electric output, and in principle, other sizes could be achieved. Similarly, concentrating photovoltaic or tracking, non-concentrating photovoltaic systems can be developed for practically any required size. Trough collectors are usually joined together for lengths of tens to hundreds of feet length. In general, the drive unit power rating and reduction ratio is similar to that for heliostats and dish concentrators.
Therefore, another desirable capability related to cost of the drive unit, is its ability to be used with visually no modifications for a multitude of applications. This would increase the production rate, and thereby reduce the cost. In addition, the drive unit should be such that it can be relatively easily modified for use with a range of sizes and still be cost effective.
It is, therefore, desirable to provide a low cost, high performance drive unit primarily for application to a high concentration solar central receiver, but which has application across a wide range of systems. It is also desirable for the azimuth drive unit to be useful for trough concentrator systems requiring large angular range.
The above and other objectives are provided by an azimuth drive unit for a solar system in accordance with the present invention. The azimuth drive unit of the present invention includes an input shaft for receiving an input torque, and a multi-stage chain and sprocket configuration coupled to the input shaft. The sprocket configuration converts the input torque into an output torque, and an output shaft is coupled to the sprocket configuration for applying the output torque to a solar reflector. Use of a multi-stage sprocket configuration eliminates destructive backlash forces, dampens shock, prevents reflector oscillations and provides redundancy while reducing costs. The multi-stage sprocket configuration also allows the drive unit to be used in a wide range of systems.
Further in accordance with the present invention, a multi-stage sprocket configuration for a solar power system azimuth drive unit having an input torque is provided. The sprocket configuration includes a plurality of sprockets, and a plurality of chains linking the sprockets such that the input torque is converted into output torque. The sprocket configuration further includes a tensioning system contacting one or more of the chains and a housing of the drive unit. The tensioning system applies a tension force to the contacted chains such that backlash in the sprocket configuration is reduced. The tensioning system offers the ability to achieve zero backlash under operational load conditions, as well as the ability to dampen any severe shock loads that exceed the operational loads.
The present invention also provides a method for applying torque to a solar reflector. The method includes the steps of applying an input torque to an input shaft, and coupling a multi-stage sprocket configuration to the input shaft. The sprocket configuration converts the input torque into an output torque. The method further includes the step of coupling an output shaft to the sprocket configuration, where the output hub, attached to the housing, and mounted on the output shaft, applies the output torque to the solar reflector.